Dermal Sense- Organs of the Crustacea. 307 



II. Histological Structure of the Nerve-end 

 ArPARATus OF THE Sensory Hairs of Crustacea. 



Tlie histology of tlie nerve-end apparatus of tlie various 

 sensory hairs, wliether olfactory tubes or tactile hairs (smooth, 

 half-feathered, completely feathered, or toothed), is essen- 

 tially the same, and corresponds most minutely Avith what I 

 have previously described for Myriapods and Insects. My 

 interpretation of the finer structure of the nerve-end apparatus 

 of the sensory hairs of Arthropods differs somewhat from the 

 statements of other authors. 



In the Crustacea, beneath the base of each capilliform struc- 

 ture serving a sensory function, there lies a group of cells 

 which is connected with a nerve ; these cells are termed a 

 ganglion by authors ; but since they are manifestly the per- 

 cipient epithelial cells, I prefer to term them sense-cells, 

 without, however, intending thereby to insist on a strict 

 physiological distinction between ganglion- and sense-cells. 

 In very rare cases only, e. g. in the whole of the sensory 

 hairs of the cirriform feet of Lepas^ I found beneath the hair 

 only a single bipolar sense-cell, of relatively large size and 

 elongate in form, with a roundish nucleus which considerably 

 exceeded the nuclei of the cells of the hypodermis in size. 

 According to the usually accepted view, the nerve which is 

 connected with the ganglion-cells is supposed to traverse the 

 entire length of the ganglion and then enter the sensory 

 hair. I have been able in a very large number of cases, e. g. 

 in the olfactory tubes of Astacus *, to convince myself with 

 absolute certainty of the fact that the nerve in no way passes 

 through the group of sense-cells, so that the sense-cells are 

 attaclied to the nerve-fibrils much as the grapes in a bunch ; 

 on the contrary, the nerve splits up beneath the group of 

 sense-cells and gives off a fibril to each cell. In the anterior 

 or distal region of the group of sense-cells I then distinctly 

 saw the way in which the protoplasmic prolongations of the 

 various cells unite into a finely streaked bundle, the terminal 

 cord^ which actually enters the hair, while its fibrillate nature 

 can often be distinctly recognized right to the tip of that 

 structure. Strictly speaking therefore the sensory hair does 

 not contain a true nerve, but rather the united prolongations 

 of sensitive epithelial cells j it follows therefore that we can 

 scarcely speak of a true axis-cylinder or axis-fibre. The 



* An olfactory tube oi Astacus, \\At\\ the nerve-end apparatus belongino- 

 thereto, has akeady been described and figured by me in my previous 

 publication (Archiv f. mikr. Anat. 27 Bd., 1886). 



